pH Values of "C", "D", and "E" Referenced to Buffer Standards

The original pH data from the laboratories, as well as the result obtained by mathematically recalibrating the original data with the individual laboratory's solution B measurements, have been quantitatively evaluated. Table 2 gives the original data and table 4 the recalibrated data. One-way analyses of variance were run on the original and the recalibrated data for non-buffer solutions C through G. The analyses of variance calculations gives within- and between-laboratory components of variance. The square roots of these quantities are called the within- and between-laboratory components of standard deviation, and are reported in table 5.

Examination of table 5 results shows the withinlaboratory components of standard deviation to be about 0.015 pH units, and to be approximately equal for the original and the recalibrated data sets. The betweenlaboratory components of standard deviation, however, are markedly different between the original and the recalibrated data sets. As expected, the recalibrations with solution B have resulted in greatly improved betweenlaboratory precisions. The component of betweenlaboratory standard deviation is about 0.07 pH units for the original data, and is about 0.03 for the recalibrated

data. The reader may evaluate the stability of the precision estimates by comparing the results from the identical solutions C and G.

Conclusion

The results of this interlaboratory test show that the use of dilute strong acid solutions of known and independently verifiable pH (e.g., solution B) to standardize pH instrumentation greatly improves betweenlaboratory precision in the measurement of pH in low ionic strength solutions, such as acidic rainwater. Thus, the use of such solutions by all those involved in network monitoring of wet deposition is recommended to improve the intercomparability of the measurements as a function of time and location. To aid in this endeavor, Standard Reference Material, SRM 2694, "Simulated Rainwater" has been prepared and analyzed by our laboratory and is available through the Office of Standard Reference Materials of NBS. SRM 2694 consists of a set of two 50 mL solutions in polyethylene bottles. The nominal pH of Level I is 4.3 and that of Level II is 3.6. The acidity and the specific conductance have also been determined, as well as several of the major cations and anions commonly found in rainwater. Guidelines for the measurement of pH in rainwater with the use of these materials are included with the Certificate of Analysis.

[1] Koch, W. F., and G. Marinenko, Simulated Precipitation Reference Materials: Measurement of pH and Acidity, Special Technical Publication 823, American Society for Testing and Materials, Philadelphia, PA, pp. 10–17 (1983).

[2] Marinenko, G., and W. F. Koch, A Critical Review of Measurement Practices for the Determination of pH and Acidity of Atmospheric Precipitation, Environment International 10, 315–319 (1984).

[3] Durst, R. A., Standard Reference Materials: Standardization of pH Measurement, Special Publication 260-53, 48 pages (Dec. 1985).

[4] Wu, Y. C.; W. F. Koch, and G. Marinenko, A Report on the National Bureau of Standards pH Standards, J. Res. Natl. Bur. Stand. (U.S.), 89 (5), 395–400 (1984).

Appendix A

4.004 at 25.0 °C, 0.05 molal], or equivalent. Refer to ASTM D1293, “Standard Test Methods for pH of Water" for guidance. Record the value on the enclosed data sheet.

With the slope adjustment of the meter set at 100%, check the Nerstian response of the pH measurement system with a second buffer, SRM 186lc/186llc, potassium dihydrogen phosphate/ disodium hydrogen phosphate [pH(S) 6.863 at 25.0 °C] or equivalent. Refer to ASTM D1293 for guidance. If the reading for the second buffer is not within 0.03 pH units of the prescribed value, recheck the calibration of the system. DO NOT CONTINUE with the interlaboratory test until the conditions for calibration and Nernstian response have been satisfied. See Note 4. If the reading for the second buffer is within 0.03 pH units of the prescribed value, record the value and continue. Rinse the electrodes thoroughly with distilled water (ASTM Type II or better). Remove drops of water on the electrode by blotting gently (Do Not Rub!) with a clean lab tissue.

Insert the electrodes into a clean beaker (10–20 mL capacity) containing a portion (10-20 mL) of solution A (or subsequent solution). Be certain that the reference junction and glass bulb are completely immersed. Do not insert the electrodes directly into the polyethylene bottles.

Stir or swirl the solution to ensure homogeneity and contact with the electrodes.

Allow the solution to settle to a quiescent state (approx 30 seconds) Record the pH value after the reading has stabilized. (Some systems may require five minutes or more to stabilize ) If drifting persists, record the value after 10 minutes in the quiescent state and rote this fact in the